The application of unrelated donor bone marrow transplantation is limited by availability of donors, an expensive, time-consuming process of working up donors, and a high incidence of severe side effects arising from donor- recipient immunological incompatibility. Recent studies suggest that use of unrelated cord blood-derived hematopoietic stem cells for human pediatric transplantation is associated with less severe graft-versus-host disease (GVHD) than seen with marrow from adult unrelated donors. However, graft failures have been observed and concern remains that cell doses from cord blood will be inadequate for adult transplantation. In vitro studies have shown greater potential for ex vivo expansion and gene transduction of cord blood hematopoietic progenitors than with adult marrow or peripheral blood and suggest that cord blood is a superior product for such manipulation. The in vivo application of these observations will be tested in a well characterized large animal model. In vitro characteristics of canine cord blood cells will be compared to those of human cells including colony forming activities, cell surface phenotypes and cell cycle characteristics. Studies will then evaluate and optimize conditions for in vitro expansion of cord blood cells. The hypothesis that cord blood transplants will engraft from subtherapeutic cell doses after ex vivo manipulation and engraft across major immunologic barriers with reduced GVHD will be tested in transplants of histocompatible siblings and mismatched unrelated dogs. The hypothesis that cord blood stem cells can be more readily transduced with retroviral constructs for gene therapy than adult stem cells will be tested by in vitro colony assays and by transplantation studies in conjunction with Project 7. The proposed studies should help define whether ex vivo expansion of cord blood can enhance engraftment after allogeneic transplantation and help determine whether cord blood transplantation for adults can safely supplant the use of marrow from adult unrelated donors.